This invention relates to borehole logging devices of the type wherein electromagnetic energy is used for measuring properties of formations surrounding a borehole and, more particularly, to improvements in such devices to reduce the effect of spurious modes of the electromagnetic energy.
Induction logging has been employed for many years for measuring the conductivity of subsurface earth formations surrounding an earth borehole. In conventional induction logging a number of coils are wound on a mandrel. One or more transmitter coils are energized by an alternating current at a frequency such as 20 KHz. The resultant oscillating magnetic field causes induction of circulating currents in the formations which are substantially proportional to its conductivity. These currents, in turn, cause a voltage to be induced in receiver coils, and the conductivity of the formations is determined from the induced voltage. Spurious modes of electromagnetic energy can arise and cause problems in conventional induction logging, but such problems are considerably more pronounced in logging devices operating at higher frequencies.
In recent years logging systems have been proposed for employing radio frequency electromagnetic energy in the range between about 10 MHz and 100 MHz to determine both the dielectric constant and the conductivity of formations surrounding a borehole. In this frequency range, dielectric constant and conductivity both have a substantial effect upon the propagation constant of electromagnetic energy propagating in the formations, so measurements of attenuation and phase can be used for solution of simultaneous equations to determine the dielectric constant and/or conductivity of formations through which the electromagnetic energy has passed. A device of this type is the so-called deep propagation tool, an embodiment of which is described in U.S. Pat. No. 4,209,747. This device includes a transmitting antenna, a "close" receiver antenna pair, and a "far" receiver antenna pair. Each of the transmitter and receiver antennas are coils wound in insulating media mounted on a metal cylindrical pipe which carries wiring to and/or from the coils. Metal "end caps" are generally provided at the opposing ends of the device. Briefly, operation of the deep propagation logging device involves energizing the transmitter to emit electromagnetic energy at a frequency suitable for determination of both the electrical conductivity and the electrical permittivity of the surrounding formations. A portion of the electromagnetic energy which has traveled through the formations is received at the close and far differential receiver pairs. The signals detected at the far receiver pair are used to determine the phase shift of electromagnetic energy that has passed through the formations, and the signals detected at the close receiver pair are used to determine relative attenuation of the electromagnetic energy. The phase shift and attenuation are then employed to obtain electrical permittivity and electrical conductivity of the formations.
The relatively high frequency electromagnetic energy that is used to obtain the substantial displacement currents needed to measure dielectric properties of the formations attenuates quickly as it travels through the formations. The receivers are typically spaced a substantial distance from the transmitter to obtain a significant depth of investigation into the formations. Accordingly, the signal levels received at the receivers (particularly the more distant of the far receiver pair) tend to be weak, especially in relatively conductive (lossy) formations. Since it is necessary to obtain accurate measurements of the relative attenuation and phase at the receivers, it is desirable to have the signal-to-noise ratio at said receivers be as high as possible. When the received signal is weak, however, as is often the case, the amount of spurious or interfering signal (i.e., "noise") is a limiting factor on the measurement accuracy of the logging device.
It is an object of the present invention to improve operation in the described type of logging devices, and in induction logging devices in general, by substantially reducing deleterious noise effects.